1. Field of the Invention
The field of this invention in general to semiconductor processing. More particularly, the field of the invention relates to systems and methods for simultaneously etching films deposited on both sides of the a semiconductor substrate.
2. Background
In semiconductor manufacturing, films may be grown and/or deposited on both sides of a wafer or other semiconductor substrate. These films often need to be removed after device structures have been defined by patterning and etching on one side of the wafer. The side of the wafer on which the devices are formed is typically referred to as the top, or front side of the wafer. The other side of the wafer, the side that does not have the devices formed thereon, may be referred to as the backside of the wafer.
One method for simultaneously removing these films from both sides of the wafer involves a wet etch bath. For example, silicon nitride films made in a low pressure chemical vapor deposition reactor have been used as a masking layer for selective oxidation in local oxidation of silicon (LOCOS) and shallow trench isolation (STI) technology. The nitride films may be removed using a hot phosphoric acid wet etch bath. Such wet baths can remove the films from both sides of a wafer simultaneously while offering other good characteristics such as high etch rates and high selectivity to any underlying films. However, a wet bath may become more disadvantageous as wafer sizes increase and the sizes of devices decrease. These disadvantages may include, among other things, the following: (i) chemical disposal may be expensive because frequent chemical change is required to guarantee stable process results; (ii) vapor from a typical wet bath is harmful to the operator""s health and environmental isolation of the chemical for operation may be expensive; (iii) etch baths may leave undesirable residues and particles that adversely impact yield of small geometry devices; (iv) wet etches may not be suitable for the high aspect ratios common to newer and smaller submicron structures, (v) a single wet etch bath may be unable to etch multilayer films, whereas a dry etch may be able to remove multilayer films in one process chamber, and (vi) typically a whole cassette (or several cassettes) of wafers are at risk with wet etch and the cassette(s) could be lost if something goes wrong with the process or the robotics; dry etch processes are usually done one or two wafers at a time.
One type of dry etch system that may be used for two-sided etching is the barrel etcher. This type of etcher has been used for photoresist stripping, but may have disadvantages such as a strong wafer-to-wafer or within-wafer loading effect, poor etching uniformity, susceptibility to particulate generation due to the lack of a vacuum load lock, and risk of loss of a full cassette or multiple cassettes of wafers if the process fails.
As wafer size increases and device features decrease, process requirements become more stringent. As a result, what is desired is an isotropic etcher which provides improved process performance and control, in-situ end point monitoring so that the process duration is correctly controlled for each wafer, and reduced particle contamination.
Since typical plasma etchers etch one side of a wafer at a time, special care must be taken to remove the unwanted material from the backside without allowing sensitive materials on the front side to be damaged. One approach to is to coat the front side of the wafer with photoresist and turn it upside down to etch the backside. Another approach is to put the plasma source beneath the wafer. In this latter technique a non-reactive gas may be flowed across the front side of the wafer for protection of the device features while the backside is etched. After the backside etch is completed, the front side may be etched in a conventional manner. Although this approach can be effective, throughput is limited because the front and backside etches are not done simultaneously.
Simultaneous two-sided etching of a wafer is difficult to carry out in many conventional plasma etchers, particularly when the device features are sensitive to process conditions. For instance, in LOCOS nitride processes, a film of silicon nitride (which may be about 1,000 angstroms thick) may be deposited on both sides of the wafer. The film becomes a mask and is used to define where field oxide will be grown by protecting surfaces on the front side of the wafer that eventually will become the contacts and gates of transistors. The exposed and/or underlying areas may be very sensitive to etching of the silicon, ion bombardment or ultraviolet light, any of which contributes to damage causing mechanisms. The etch process to remove silicon nitride must be highly selective, with an etch rate selectivity of silicon nitride to silicon oxide of about 30:1 or greater. Other sensitive processes include shallow trench isolation which may require nitride to oxide selectivities as great as 100:1, depending on whether the front or back side is etched faster. In some embodiments where the backside of the wafer could be etched faster than the front the selectivity might only need to be about 40:1 (as in the LOCOS process).
In a two-sided etch where both sides of a wafer are etched simultaneously, it is sometimes desirable to have both the front side and backside etch rates approximately in proportion to the average thicknesses to be etched on front and backsides of the wafer. Otherwise, one side would have to be overetched longer than the other and the selectivity to other layers on that side would have to be increased even further to avoid damage. In some applications, it can be desirable to have the backside etch rate faster than the front side etch rate, as much as three times faster in shallow trench isolation or possibly other new processes. Obtaining this kind of control in a single wafer plasma etcher is difficult when the reactant is provided to the wafer from only one side (typically the front side). While plasma etchers such as reactive ion etchers may be able to perform a two-sided etch by processing wafers on lift pins, the hardware underneath the wafer may limit the backside etch rate or result in non-uniformities. Also, if the wafer is exposed directly to the plasma, ion bombardment, UV exposure, and charging effects may damage sensitive features.
What is desired are systems and methods for a simultaneous two-sided etching one or more of the following with: (i) reduced damage from ion bombardment and ultraviolet light; (ii) high selectivity to underlying layers; (iii) high back to front etch rate ratio; (iv) good etch rate uniformity on both sides of the wafer; and (v) a high rate of etching on both sides of the wafer in a single processing step to allow for high throughput. It is particularly desirable to have an appropriate of each of these features in combination.
One aspect of the present invention provides a reactor system for etching two sides of a semiconductor substrate. An exemplary reactor system comprises a generation chamber for producing reactive species, a gas inlet for providing gas to the generation chamber, a process chamber within which a semiconductor substrate is processed, and a gas outlet for exhausting gas from the process chamber. Within this reactor system the gas flows generally from the generation chamber to the process chamber and is then exhausted. Within the process chamber is a support for the substrate which exposes both front and backsides of the substrate, and a diverter which: (a) allows a first portion of the flow of reactive species from the generation chamber to flow to one side of the substrate; and (b) diverts a second portion of the flow of reactive species around the diverter and the substrate to flow to the other side of the substrate.
Another aspect of the present invention provides a diverter configured so that a first portion of the flow of reactive species either passes through the diverter, or to some degree goes around it, or both, to reach the region between the diverter and a first side of the wafer. In this context, the xe2x80x9cfirst sidexe2x80x9d of the wafer is the side of the wafer substantially facing the diverter, and thus the first side may be either the front or the backside of the wafer. A second portion of the flow of reactive species bypasses the diverter and avoids the region between the diverter and the first side of the wafer. This is achieved, in part, because some of the gas flow from the generation region is diverted in a direction substantially parallel to the diverter so that it does not penetrate the region between the first side of the wafer and the diverter. Both the first and second portions of the stream of gaseous species flow past the second side of the wafer. In this context, the term xe2x80x9csecond sidexe2x80x9d is the side of the wafer that is substantially facing away from the diverter. In flowing past the first and second sides of the wafer the reactive gaseous species cause some of the materials on the wafer surfaces, both first and second sides, to be etched in an adequately selective and uniform manner.
Another aspect of the present invention provides a method for processing a substrate on a set of pins to sufficiently remove the material between the pins and the wafer without having to move the wafer to a second set of pins wherein the area contact of the second set does not overlap the area contact of the first set.
A further aspect of the present invention provides method for two-sided etch that allows the following process results to be balanced in a desired manner: (i) etch rate uniformity and etch rate selectivity on the first side of the wafer, (ii) first side to second side etch rate ratio, and (iii) second side etch rate uniformity.
Aspects of the present invention allow for simultaneous etching of films on both sides of a semiconductor substrate. Aspects of the invention may be used with virtually any source of reactive species, including those utilizing plasma sources, providing species to a downstream wafer-processing chamber. In one embodiment, the substrate is supported by three sharp-tipped pins so that both sides of the substrate are exposed to reactive species for etching. A first portion of the flow of reactive species generated in the reactive species source is provided to one side (this may be the front side) of the substrate for reaction. A second portion of the flow is directed around substrate to the other side (which may be the backside) where it is then pumped out of the process chamber. To provide a sufficient and appropriate flow of reactive species to the backside of the substrate, a gas flow diverter is placed between the reactive species generation chamber and the substrate to reduce the flow to the front side. This causes some of the reactive species to flow to the backside of the wafer without having passed adjacent to the front side of the wafer. A flow restricter may then be placed between the substrate and the exhaust port to increase the residence time and likelihood of reactive species adjacent to the second (may be the backside) of the wafer.
Various types of diverters and combinations of diverters, baffles, and restricters can be used in embodiments of the present invention to achieve the desired process results. The diverter used in some embodiments may comprise one or more plates, and at least one of those plates may contain a hole or pattern of holes. The diameter of the diverter, hole pattern, hole size, and distance of separation between the diverter and the substrate may be adjusted to achieve the desired process results. In one embodiment, the diverter may also screen the line of sight from the plasma to the substrate, minimizing UV radiation and charging damage, thus contributing to process optimization. In the case where the diverter consists of more than one plate, exemplary embodiments may include configurations in which that part of the diverter closest to the wafer has holes for gaseous species to flow through to the wafer. This multi-plate diverter may be used to prevent line-of sight to the wafer from the generation region, which may contain a plasma.
Another aspect of the present invention provides for a flow restricter located downstream from the wafer with respect to the flow of reactive gases. As with the diverter, the flow restricter may comprise a plate with a hole pattern (or other mechanism to restrict gas flow) where the holes provide an exit path to the exhaust pump. The hole size and shape may be selected to be both small enough to provide a long residence time for reactive species, but also large enough to allow for sufficient gas flow conductance. The distance of the plate or other restriction mechanism to the substrate with respect to the substrate diameter may be selected to achieve the desired backside etch rate and uniformity. The shape of the bottom plate or other mechanism may also be varied to optimize process results.
It is an advantage of the foregoing and other aspects of the present invention that high selectivity, low ion damage, good etch rate (e.g., 200 to 1,000 angstroms per minute) and good uniformity may be achieved in a simultaneous two-sided etch process.